Photometer.



l A BLONDEL.

PHOTOMETER.

. 1 APPLICATION FILED NOV. 4. F913. 7 LWW WWW Patentefl m. 112, 1916.

3 SHEETS-SHEET I.

JJ W M W L A. B LONDEL. PHOTOMETER.

APPLICATION map NOV. 4. 191a.

LWM WHW Pm/9mm 10m '12, 19116.

3 SHEETS-SHEET 2- A. BLONDEL.

PHOTOMETER.

APPLICATION FILED NOV-4, 1913.

3 SHEETSSHEET 3.

Pwhembed' Dee. 12, 1916.

annnn Brennan, or name, rnartcn.

rno'ronnrnn.

meagre.

Specification of Letters Patent.

Patented has. is, rare.

Application filed November 4., ma. Serial Ito. 799,15t.

To all whom it may concern.

Be it known that I, Arman Bnonnnn, a citizen of the French Republic, residing at Paris, France, have invented a new and useful Photometer; and I do hereby declare the following to be a full clear, and exact description of the same.

This inventidn has for its object a portable photometer capable of measuring without difficulty by means of a luminous comparing surface, the apparent brightness of surfaces which are illuminated by any luminoussource, and of determining if necessary the intensity of. said source. For this purpose various apparatus have been constructed which make use for varymg the intensity of the comparing illumination,

. paring either of the-displacement of the auxiliary source orlthe displacement of a screen illuminated by the latter.

- According to the present invention the operator views the surface to be photometered through the transparent portion of a half-reflecting and half-transparent surface. The reflecting portion receives the. light either from a comparing source or from a compar ng screen illuminated by sa1d source.

It varies the-illumination of the reflecting 3'0 portion either by means of -an apparatus having a slit of variable widths which varies the dimension that is employed of the source of light, either by displacing the comparing.

screen by means of a translating mechanlsm,

or by varying the relative direction of two Nicol prisms placed between the source and the screen or by any other known method. The whole is contained in a box which contains also a measuring instrument to allow of controlling the intensity or the voltage of the current serving to illuminate the comluminous source.

The present invention 'is illustrated by way of example in the accompanying drawings, in which:- i

'Figure 1 is a longitudinal section of the apparatus on the line A,-B of Fig. 2. Fig.

2 is a plan of the apparatus with the cover removed- Fig. 3' is a cross section of the apparatus on the line C--D of Fig. 2. Fig. 3 shows what is seen by an observer looking through the eye hole for making the measurement. Fig. 4 is a plan of the apparatus when closed. Figs. 5 and 6 illustrate a double catseye device employed in'the modification of the apparatus. Figs. 7 and 8 are respectively a vertical'section and a plan of a modification. Fig. 9 and 10. are similar figures of another modification.

The portable'photometer shown in Figs. 1, 2, 3, and 4, consists of a box 1 adapted to be closed by means of a' cover 2. Inside the .box 1 is an arrangement of apparatus allowing of comparing the luminous source to be measured with a source of uniformbrightness whose surface is varied in known ratios. The light of'the source to be'photometered enters the apparatus through the aperture 3 (Fig. 2). This aperture may or may not'be provided with an opal glass 4 carried by the mount 5, which is screwed in the aperture 3 according as it isdesired to measure the intensity of a, luminous source or that of a brightness or illumination.

The half-reflecting and half-transparent surface is constituted by the separating surface if two rectangular triangular prisms 6, uxtaposed on the Lummerrodhun system, one of which bears upon the common surface silvered parts 30 (Fig 3); while the remainder 28 of the surface is transpan' cut. The comparing source consists in the I example shown of a glow lamp 8. The current supplied by a small portable storage battery enters at the terminals 9,passes through the rheostat 10, the-lamp 8 and the amperemeter 11.

The intensity of the current in the lamp is regulated by operating. the movable contact 12 of the rheostat 10; The lamp 8 is a straight-filament lamp. The filament 5t is arranged horizontally at right angles to the direction of the luminous rays projected duced by means of the diaphragm consti-- by an aperture 13 of regulable width tuted situated in front of the filament 5t. The width of the aperture 13 is determinedby the position of a movable shutter 14 operated bymeans of a. screw 15 through the medium of a sliding sleeve 16 bearinglgraduated divisions drawn on, a linepara in a fixed plate 52 parallelto the/axis of the screw 15. A fixed rule 17 situated transversely to the box and parallel to the axis of the screw 15, bears one or more fixed reglel tothe axis of'the screw 15. This shutter slides in' front. ofa wide-horizontal slit 5]]. formed 1 displacement of the sleeve 16 and the shutter 14.

In order to increase the accuracy of the reading ofi', there is fixed on the spindle of the screw a cylinder 18 bearing on its circumference a series of divisions. The pitch of the screw 15 is made such that it shall be equal to the spacing of the divisions marked on the sleeve 16. The divisions on the cylinder 18 enable the fractions of a turn of the screw to be determined. The diaphragm constituted bythe aperture 13 and the shutter 14 may for example be so formed as to allow of a displacement of the shutter equal to five times the pitch of the screw, and the micrometric cylinder may have a hundred divisions, so that readings up to 1/500 can be made without difficulty.

The observer who takes the measurements takes a sight generally through the eye-hole 19 formed in the side of the box 1 opposite the aperture 3.. In some cases the observer may also look through an eye-hole 20 formed in the cover 2 (see Fig. 1). In such a case the luminous rays issuing from the aperture 3, and from the comparing luminous source are reflected by a mirror 21 mounted ona shutter 22 that is movable on the axle 23. A knob 25, allows of fixing the shutter 22 either in its horizontal position or at an angle of degrees. For the purpose of brin 'ng the eye to the minimum distance for distinct vision of the surfaces to be examined, a telescopic tube 26, provided if desired with an eye piece 27, may be screwed on to the eye holes 19 or 20.

The path of the light rays through the apparatus is as follows :The rays coming from the outside, pass first through the aperture 3, then through the unsilvered portion 28 of the prism '6, continue on leaving the said prism in a path parallel to the long side of the box, and then'stri'ke the eye of the observer, either directly when his eye is in the position 7, or indirectly and by reflection on the mirror 21 when the eye is in the position 24. The rays emitted by the comparing source. (see arrows in Fig. 2), the slit 13, strike a silvered mirror 29 which is located, when the box is horizontal, in a vertical plane arranged at 45 degrees to the side of the box, are reflected by this mirror, strike the silvered portions 30 ofthe prism 6 by which they arereflected, and thence follow a path parallel to that of the rays coming from the outside. An. angle piece 31 fixed under the cover 2, revents the rays of the source of comparison from passing to the other side of the diaphragm 13, otherwise than through the adjustable slit.

The cover 2 ofthe apparatus has three lenses (Fig. 4). The lenses 32 and 33 serve for-reading the divisions on the sleeve 16 and the eylinder 18. .The lens 34 serves for reading the divisions'of the amperemeter 11 for the purpose of verifying the constancy of the current feeding the comparing lamp.

The divisions of the sleeve 17 of the cylinder 18 and of the amperemeter 11 are illuminated by the light of the lamp 8. The

device of the silvered part 30 shown in Fig. 3 may also be replaced with advantage by a silvered portion in the form of a vertical strip of 1 to 4 mm. in width.

The diaphragm may, as shown in Figs. 5 and 6, have a form different from that shown in Fig. 2. In this second modificathrough the unsilvered portions 28 of the prism 6 the luminous surface of which he desires to measure the illumination. \Vhen the current is not passing through the lamp, the observer will see, as shown in Fig. 3, a series of dark spots'corresponding to the silvered portions 30 of the prism contrasting with the clear background formed by the transparent portions.

On allowing the current to pass through the lamp the rays emitted by the latter will pass through the slit 13, will be reflected on the mirror 29, and will strike the silvered portions of the prism which they illuminate. These silvered portions, instead of appearing as dark spots, will appear to be illuminated, and their illumination may be greater or less than that of the back ground formed by the transparent portions 28, according to the intensity of the comparing source. By

operating the screw 15 of the diaphragm 13 the illumination of the silvered portions may be made equal to that of the unsilvered portions in such a manner that the eye shall perceive only a single uniformly illuminated spot.

To enable measurements to be made in the case where the source whose illuminations is to be measured, is not very luminous, a series of absorbing glasses are provided which are placed in the path of the rays emitted by the source of comparison, for instance at 44 between the mirror 29 and the prism. Similarly absorbing glasses may beplaced at 45 and 46 in the path of the light rays emitted by the source, the intensity of whose luminosity it is desired to measure. These glasses may be fixed in mounts 47 and 48 and be moved into position by means of rods 49 and 50. In this manner a uniformly illuminated surface is neonate presented to the eye in all cases. The readmg of the micrometric divisions indicating the Width of the aperture 13 or of the catseye, then allows of determining without dilliculty the luminous intensity of the source which is being investigated.

I When it is desired to employ the apparatus for measuring the apparent brightness of an illuminated surface, the observer looks directly at the said surface through the sighting telescope in such a manner as to see the prism 6 projected upon the surface which is being investigated. The apparatus is calibrated or standardized by measuring the aperture in the diaphragm or the catseyedirecting upon the silvered portions 30 a brightness apparently equal to that directed upon the background 28 by a sheet of white paper placed at a known distance from a light of known intensity. It is of advantage to regulate the intensity ofthe current in the lamp so that the constant of the apparatus thus obtained for the aper ture in the diaphragm shall be a round numher. For instance, so that a sheet of white paper receiving an illuminationof lux shall be made equal as regards illumination upon the surfaces 28 and 30 of the prisms 6 through the aperture of a cats-eye equal to 500 divisions.

For measuring the intensities of sources of light, the opal glass 4 is placed upon the mount 5, and this opal glass is directed at right angles to the direction of the rays emitted by the source of light which isbeing investigated, after having moved the apparatus to a known distance from this meter distance from the opal g source. I

The aperture of the diaphragm is varied so as to produce equal illuminations upon the surfaces 28 and 30. Then this source oflight is replaced by a source of known intensity placed at the same distance and the ratio between the apertures of the cats-eye in the two cases will be a measure of the ratio between the luminous intensities. Fun ther the constants of the apparatus may be established by measuring the number of divisions which correspond for instance to. a source of light of 10 candles laced at one ass. For the of measuring illuminations in the observer views through the purpose streets,

prism a surface of comparison which is always the same, .for instance a screen of dull-enameled sheet metal or a sheet of dull 'Bristol'board placed at the very spot it is desired to measure the illumination; this sheet being arranged horizontally -or verti cally according to the purpose in view.

The apparatus forming the subject of the invention is diderentiated from the known apparatus in that the distance of the. source of comparisonfrom the screen on which the two illuminations are compared is fixed, and

that may be desired. The measuring operation is further extremely rapid, so that the apparatus may be used for instance by pho- I tographers for determining the duration of exposure which is of great advantage for color photography. Figs. 7 and 8 show in vertical and horizontal sections a modification of the foregoing apparatus in which the variation of the illumination of the source of comparison instead of being produced by varying the opening of a slit, is produced by the displacement ofthe comparing screen.- In these figures the comparmv screen 60 may 7 be displaced by any suitable means for instance, by means of an endless wire 61 passing over two pulleys 62, 63 for drawing the carriage 64 that carries the screen. 7 This carriage is guided by two slide rods 65. By this means the distance of the screen 60 may be varied for instance from 1 to 10, relatively to the inner lamp 6,6'which serves as a fixed source of comparison. This operaspec'ts the construction of the photometer is identical with 1 and 2.

Fi s. Qand 10' show similarly in vertical and orlzontal' tions, another modification in which the intensity of illumination produced by the lamp 66 which-is fixed, upon a comparin screen 60 which is likewise fixed, is mo ified at will by using two Nicol prisms 71 prisms, 72, for instance, is that iscapable of turningbon: its axis, and whose rotation is efi'ected a worm 7 3 actuatedby means of a milled hole 74. The rotation of the prism 72 may be measured in any suitable'manner. For instance the face 7 5 of the rotary tube may he graduated and t ammate arranged in front of this graduation.- Or, the worm maybe provided and 72., Une of thesefixed in a tube that of the foregoing Figs.

It is to be understood that this invention is not limited to the details described and shown. For instance, in its applications where a smaller degree of accuracy is required, the prism 6 may be, replaced by a glass plate arranged at 45 degrees. The source may be as in the case shown, a glow lamp having a straight filament made of transparent or matted glass (dull surface)- or it may be an opal globe in the center of which is a small glow lamp.

. The amperemeter serving to calibrate, may also be replaced by a voltmeter measuring the potential at the terminals of the said lamp. The box may be provided with two threaded sockets for enabling the apparatus to be fixed upon a stand (for instance, the stand of a photographic apparatus). The said box may also be provided with a hook or with a slideway for fixing the storage battery thereto.

Having fully described my invention, what I claim and desire to secure by Letters Patent is:

1. In a photometer, the combination of an incandescent lamp having a straight luminous body; and a part provided with an adjustable slit in close proximity to said body; alnd means for adjusting the width of the s it.

2. In a photometer, the combination of an incandescent lamp serving as a source of light with which the light to be examined is compared and comprising a straight luminous body of small diameter; and a pair of shutters in close proximity to the lamp; and means for adjusting the width of the slit formed between the shutters.

3. In a photometer, the combination of an electric incandescent lamp comprising a cylindrical bulb of small diameter and a straight light 'ving body of small diameter and locate axially thereof; a pair of relatively movable shutters located in close proximity to said bulb; and a micrometer screw adapted to engage one of the shutters for changing at will the width of the slit between said shutters in the direction of the length of said light giving body.

4. In a photometer, the combination of an electric incandescent lamp having a straight light giving body of small diameter; a pair of relatively movable shutters located in close proximity to said body; a micrometer screw adapted to change at will the width of the slit between said shutters in the direction of the length of said light giving body; and a screen on which the light from said body falls with a luminosity which is variable proportionally to the width of the slit.

5. In a photometer, the combination of a source of light having a straight luminous body of small diameter; a slit-diaphragm near said light; means for adjusting said slit; a screen on which the light from said source falls through said slit-diaphragm; and an optical comparison device having a reflecting portion adapted to receive rays reflected from said screen, and a transparent portion adapted to transmit the rays from the surface of which the luminosity is to be measured parallel and near to the rays reflected from the reflecting portion.

6. In a photometer, the combination of an incandescent lamp serving as a source of light for comparison with the light to be examined, and comprising a straight luminous filament of small diameter; and means forming an adjustable slit in close proximity to said filament; means for accurately adjusting and measuring the width of the slit; and a galvanometer and a rheostat connected in series with said lamp for varying and verifying the current traversing said In testimony whereof, I have signed my name to this specification in the presence of two subscribing witnesses.

' ANDRE BLONDEL.

Witnesses:

HENRI BoENzA, HANSON C. Coxn. 

